From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pg0-f69.google.com (mail-pg0-f69.google.com [74.125.83.69]) by kanga.kvack.org (Postfix) with ESMTP id 8D2906B0253 for ; Thu, 30 Nov 2017 19:09:22 -0500 (EST) Received: by mail-pg0-f69.google.com with SMTP id w22so5236067pge.10 for ; Thu, 30 Nov 2017 16:09:22 -0800 (PST) Received: from lgeamrelo11.lge.com (LGEAMRELO11.lge.com. [156.147.23.51]) by mx.google.com with ESMTP id b5si3853870pgr.120.2017.11.30.16.09.20 for ; Thu, 30 Nov 2017 16:09:21 -0800 (PST) Date: Fri, 1 Dec 2017 09:09:19 +0900 From: Minchan Kim Subject: Re: [PATCH] list_lru: Prefetch neighboring list entries before acquiring lock Message-ID: <20171201000919.GA4439@bbox> References: <1511965054-6328-1-git-send-email-longman@redhat.com> <20171129135319.ab078fbed566be8fc90c92ec@linux-foundation.org> <20171130004252.GR4094@dastard> <209d1aea-2951-9d4f-5638-8bc037a6676c@redhat.com> <20171130124736.e60c75d120b74314c049c02b@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20171130124736.e60c75d120b74314c049c02b@linux-foundation.org> Sender: owner-linux-mm@kvack.org List-ID: To: Andrew Morton Cc: Waiman Long , Dave Chinner , Vladimir Davydov , Johannes Weiner , linux-kernel@vger.kernel.org, linux-mm@kvack.org On Thu, Nov 30, 2017 at 12:47:36PM -0800, Andrew Morton wrote: > On Thu, 30 Nov 2017 08:54:04 -0500 Waiman Long wrote: > > > > And, from that perspective, the racy shortcut in the proposed patch > > > is wrong, too. Prefetch is fine, but in general shortcutting list > > > empty checks outside the internal lock isn't. > > > > For the record, I add one more list_empty() check at the beginning of > > list_lru_del() in the patch for 2 purpose: > > 1. it allows the code to bail out early. > > 2. It make sure the cacheline of the list_head entry itself is loaded. > > > > Other than that, I only add a likely() qualifier to the existing > > list_empty() check within the lock critical region. > > But it sounds like Dave thinks that unlocked check should be removed? > > How does this adendum look? > > From: Andrew Morton > Subject: list_lru-prefetch-neighboring-list-entries-before-acquiring-lock-fix > > include prefetch.h, remove unlocked list_empty() test, per Dave > > Cc: Dave Chinner > Cc: Johannes Weiner > Cc: Vladimir Davydov > Cc: Waiman Long > Signed-off-by: Andrew Morton > --- > > mm/list_lru.c | 5 ++--- > 1 file changed, 2 insertions(+), 3 deletions(-) > > diff -puN mm/list_lru.c~list_lru-prefetch-neighboring-list-entries-before-acquiring-lock-fix mm/list_lru.c > --- a/mm/list_lru.c~list_lru-prefetch-neighboring-list-entries-before-acquiring-lock-fix > +++ a/mm/list_lru.c > @@ -8,6 +8,7 @@ > #include > #include > #include > +#include > #include > #include > #include > @@ -135,13 +136,11 @@ bool list_lru_del(struct list_lru *lru, > /* > * Prefetch the neighboring list entries to reduce lock hold time. > */ > - if (unlikely(list_empty(item))) > - return false; > prefetchw(item->prev); > prefetchw(item->next); > > spin_lock(&nlru->lock); > - if (likely(!list_empty(item))) { > + if (!list_empty(item)) { > l = list_lru_from_kmem(nlru, item); > list_del_init(item); > l->nr_items--; If we cannot guarantee it's likely !list_empty, prefetch with NULL pointer would be harmful by the lesson we have learned. https://lwn.net/Articles/444336/ So, with considering list_lru_del is generic library, it cannot see whether a workload makes heavy lock contentions or not. Maybe, right place for prefetching would be in caller, not in library itself. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org